I concur that the airfoiled tail is functional rather than simply cosmetic. Back when I was trying out various kitfoxes before I bought a kit, it was pretty clear that the airfoiled rudder eliminated a dead spot that the slab sided rudder had. I questioned the demo pilot at the time and he confirmed that I was not imagining the effect.

I am sure if a person could see both tails in a wind tunnel with smoke streams - the disturbance caused by the tubing without an airfoiled form would have some eddies that the airfoiled form would smooth out.Gets back to the issue that a small round rod/tube perpendicular to the airstream disturbes more air than the diameter of the tube.